The disclosed technique relates to method and systems for detecting defects in general, and to methods and systems for detecting defects in semiconductor manufacturing procedures, in particular.
A photographic mask or photomask is used in the fabrication process of manufacturing large-scale integrated (LSI) semiconductor circuits, very-large-scale integrated (VLSI) semiconductor circuits or ultra-large scale integrated (ULSI) semiconductor circuits. The manufacturing of a wafer typically calls for the use of a plurality of different photographic masks, each for imprinting a different layer on the wafer. All of these photographic masks can be reused again and again for producing many wafers. In general, a photographic mask is imprinted with a pattern, according to a predetermined pattern. It shall be appreciated by those skilled in the art that the pattern imprinted on the photographic mask should be essentially identical to the predetermined pattern. Any deviation of the photographic mask pattern from the predetermined pattern would consist a defect and shall render that photographic mask defective. It is noted that the use of a defective photographic mask shall yield defective wafers.
Hence it is important to detect such defects. When a defect is found, either the photographic mask is discarded or the defect is analyzed to determine if it can be repaired. Therefore, it is also important that the inspection system shall not declare xe2x80x9cfalse defectsxe2x80x9d (i.e., declare a defect for a non-defective photographic mask).
Semiconductor chips are becoming more and more complex and condensed in structure and smaller in size, and are thus more prone to defects. A conventional method for detecting defects often includes several different test procedures.
Defects in a photomask may take the form of holes (xe2x80x9cpinholesxe2x80x9d), xe2x80x9ccracksxe2x80x9d, spots and the like. Defects can be introduced into a mask during manufacturing, handling or transportation. Hence, a mask has to be inspected for defects after manufacturing and further routinely.
Integrated circuit photographic mask inspection methods may generally be divided into die-to-die methods and die-to-database methods. In a die-to-die inspection, two portions of the photographic mask (die patterns) which are supposed to be identical are compared, and significant differences between them indicate a probable defect in one of the die patterns. This inspection based on the assumption that there is a very low probability that the same defect shall appear in both dies. When a defect is found, a comparison with a third die pattern can indicate which of the original two die patterns is defective.
In a die-to-database method, an image representing a proper (defect-free) photographic mask is stored in a database. The database image and the actual image of the photographic mask die pattern are compared, and significant differences there between, indicate a probable defect.
U.S. Pat. No. 3,972,616 issued to Minami et al., entitled xe2x80x9cApparatus for Detecting the Defects of the Mask Pattern Using Spatial Filtering,xe2x80x9d is directed to an apparatus for die-to-database defect inspection. In the disclosed apparatus, a portion of the inspected photographic mask is illuminated simultaneously by a coherent light source and an incoherent light source, each of a different color. The image is passed through a spatial filter and projected on a screen. The spatial filter corresponds to the Fourier spectrum of the proper photographic mask pattern. The filter has wavelength selectivity, so that it filters only light of the wavelength of the coherent light. Thus, the filter causes the defect to be displayed on the screen in the color of the coherent light source. Due to its wavelength selectivity, the filter passes most of the light from the incoherent light source. Thus, the entire image of the portion of the photographic mask is displayed in the color of the incoherent light. This simultaneous display of the defect in one color and entire portion of the mask in another color enables the location of the defect in the photographic mask.
U.S. Pat. No. 4,559,603 entitled xe2x80x9cApparatus for Inspecting a Circuit Pattern Drawn on a Photomask Used in Manufacturing Large Scale Integrated Circuitsxe2x80x9d, issued to Yoshiwaka, is directed to an apparatus for die-to-database photographic mask inspection. A circuit drawn on a photomask is placed on an X-Y table, and illuminated. A linear image sensor measures the light transmitted through the photographic mask and produces analog measure signals. These signals are converted to digital signals representing a multilevel gray image. Simultaneously, a laser interferometric measuring system precisely monitors the table position relative to the photographic mask. The image sensor is synchronized with the table position measurement, using a reference clock. A reference image is generated using the table position coordinates. Since these coordinates are generally not an integer number of pixels, the reference image is shifted accordingly by convolution with a predetermined function. The reference data and the actual image data from the image sensor are compared and defects are reported.
U.S. Pat. No. 4,579,455 entitled xe2x80x9cPhotomask Inspection Apparatus and Method with Improved Defect Detection,xe2x80x9d issued to Levy et al., is directed to an apparatus and method for performing die-to-die inspection of a photomask. Two die patterns are inspected to reveal differences there between. Two digitized image sensors and two memories are used to generate pixel representations of two die patterns of a photographic mask. At any given moment, each memory contains the pixel representation of a 7xc3x977-pixel portion of the inspected die. A defect detector determines whether the two representations match. The defect detector accounts for misalignment of the dies of up to two pixels, by checking different possible alignments of the inspected portions. If none of the examined possible alignments results in a match, the system declares a defect at the corresponding location in one of the die patterns.
The article entitled xe2x80x9cAnomaly Detection through Registrationxe2x80x9d, by M. Chen et al., is directed to a method for performing registration of medical images of inspected organs of living creatures. Chen describes a method for detecting of anomalies in the brain. This method uses the fact that a normal brain is essentially symmetrical with respect to its central line. The inspection process involves registration between an image of the brain and the same image flipped with respect to the central line. The registration process involves deformation of the pixels (picture elements) of one image, which causes the images to match. Thus, if the registration between the brain image and the flipped brain image yields significant pixel deformation, it is concluded that there is an anomaly in the inspected brain.
It is an object of the disclosed technique to provide a novel method and system for detecting irregularities in an image, which overcomes the disadvantages of the prior art. In accordance with the disclosed technique, there is thus provided a method for detecting irregularities in an image. The method includes the procedures of identifying theoretically-symmetrical elements within the image, analyzing the theoretically-symmetrical elements according to their theoretical symmetry, and determining the presence of defects according to the deviation from the theoretical symmetry.